Clip machines of the kind referred to are typically used to close bags or tubular packaging (tubular casing or gut) filled with a liquid to thick pasty material or (in some cases) granular material. During such an operation, the filling is first introduced into the packaging and in the case of tubular packaging is subsequently divided into portions by means of displacement elements. Said displacement elements gather and tie the tubular casing in the radial direction and displace the filling in the constriction in the axial direction with respect to the axis of the tube. In the area where the casing is gathered and tied, a braid of casing material is formed. In the next operating cycle, one, or in the case of a double clip arrangement two clips are applied to the braided casing by means of two (or four) closing tools moved against each other. Each pair of closing tools comprises a punch and a die-plate, between which the clip is shaped during closure until the closed position is reached (the reversal point of movement). Once the packaging has been clipped, the closing tools are returned to their initial or open position.
Such a seal must meet high quality standards. On the one hand, it must not be so firm that the casing is damaged during closure. On the other hand, however, the clipped seal must be sufficiently tight to ensure that it does not slip off when the sealed sausage is subsequently processed (cooked, smoked, etc.), for example.
When the clip machine is used for many different sausage products and casings, different clips are applied that differ in respect of both strength and size (leg length and thickness of material). The closure gap must be adjusted accordingly, because this is the critical size—referred to hereinafter as the clip height—to which the clip is pressed together during closure. In the prior art, this is achieved by changing the neutral position of at least one of the closing tools. This is done empirically at first. However, the necessary precision is frequently not achieved. The users of such clip machines generally check the firmness of closure manually, and therefore very subjectively. Very often, the clip height is kept below the required height to ensure that the clip is firmly attached to the casing. As a result however, the clip machine may be subjected to much greater stresses and strains that would actually be necessary, and this can lead to greater wear and tear or even to the machine being damaged. There is also a risk that the casing is sometimes damaged during clipping, and that the filling can escape or be spoiled.
If the machine has nevertheless been adjusted in this way for a particular type of product and/or clip size, the specific clip height will be reproduced in future on the basis of a scale mounted on the clip machine. A clip machine of this kind, in the form of a benchtop machine, is known from DE-U-1933066. However, further incorrect adjustment of the machine can easily occur, especially when reproducing the setting, leading to insufficient firmness of closure occurring again and again, as well as greater wear and tear and damage to the clip machine.
Since the closing tools are moved together with closing forces in excess of several thousand Newtons (up to 15 kN) when closing a clip, a certain amount of wear and tear can be expected in all moving parts. The resultant play is not taken into account when setting the closure gap with the aid of the scale on the clip machine. In addition to the risk of incorrect adjustment of the machine, this can also result in the scale value no longer matching the actual closure gap and hence also in the actually ensuing clip height no longer corresponding to the empirically measured tightness of closure.
The users of such clip machines are thus compelled to check the tightness of closure at regular intervals, and to correct it when necessary.
A pressure limitation apparatus for such a clip machine, aimed at counteracting this nuisance, is known from EP 0 476 020 A1. In the latter, the force on the punch is reduced by means of a plunger arrangement and a pressure reservoir when a preset closing force on the punch is exceeded. Although this provides a reduced risk of the clip machine and the sausage casing being damaged, it cannot ensure that the clip is closed to a sufficient clip height.
A clip machine which includes a distance sensor for measuring the closure gap by means of a distance sensor and reference surface positioned stationary with respect to the closing tools is known from EP 0 990 733 A1. This enables a parameter that actually does correspond to the height of the closure clip to be measured. However, even this monitoring device does not exclude incorrect adjustment of the machine. For example, if an incorrect closure gap is selected for a particular clip, or if a clip from a previous closure operation is left between the closure tools due to a malfunction, measuring the closure gap of the closure tools cannot prevent the risk of a malfunction of, or even damage to the clip machine or the product or casing being closed.